1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Power consumption and power conversion by wireless communication transmitters have historically been poor. Linear amplifiers used in wireless communication networks were only a few percent efficient in the early 90's. More recently, wireless transmitters have implemented “less linear” amplifiers that use digital linearization techniques to achieve efficiencies in the 30-40% range. However, even these efficiencies are now thought to be insufficient. For example, service providers and world governments have increased pressure to decrease energy consumption and have set a target efficiency of more than 40% for future wireless transmitters. Achieving this goal would represent a revolutionary leap in efficiency for wireless transmitters, particularly in, but not limited to, the area of mobile telephony. One technique that could, in theory, achieve the target efficiencies is the so-called LINC transmitter/amplifier approach (LINC: LInear Amplification using Non linear Components).
The concept of LINC amplifiers and Chireix combiners was proposed in 1935 by H. Chireix. Analog circuits were used to construct the first implementations of a LINC amplifier. Although the theoretical foundations for the required signal processing are sound, real-world implementation of the mathematics of Chireix signal processing proved difficult and analog LINC amplifiers have to date failed to achieve the theoretical efficiencies. Using recent advantages in digital signal processing hardware, it is possible to apply the LINC methods to communication signals that conform to air interface standards such as CDMA, UMTS, LTE, OFDM, and combinations thereof. However, attempts to construct commercial implementations of a LINC amplifier for a wireless transmitter have also failed to achieve the theoretical maximum efficiencies. The failure of both the analog and digital approaches to achieve the theoretical efficiencies results, at least in part, from numerous path impairments in LINC systems. The impairments include, but are not limited to, group delay, gain, phase, flatness, and phase loading/pulling of the time varying load. Filtering has typically been used to reduce spurious emissions and current architectures rely on wideband filtering, which can reduce filter insertion losses in some cases.
FIG. 1 conceptually illustrates a conventional LINC amplifier 100 with a combiner 105 such as a Chireix combiner. An input signal 110 is provided to the amplifier 100, which includes a signal separator 115 that decomposes the input signal 110 into two constant envelope signals 120, 125. Each branch of the amplifier 100 includes a non-linear amplifying circuit 130, 135 that is used to amplify the corresponding constant envelope signals 120, 125. The amplified signals 120, 125 are then provided to the combiner 105, which combines the signals to form an amplified signal 140. In theory, the constant envelope signals 120, 125 are amplified by exactly the same gain and propagate through the two branches of the amplifier 120 with exactly the same delays and phase shifts. Consequently, when the constant envelope signals 120, 125 are combined at the combiner 105, they form an amplified signal 140 that is an exact amplified replica of the input signal 110. However, in practice each element in the LINC amplifier 100 introduces slightly different gains, delays, and/or phase shifts, which can significantly degrade the quality of the amplified signal 140.
FIGS. 2A and 2B show simulation results that illustrate the degradation in the reconstructed signal caused by a gain imbalance between two paths in a LINC amplifier. The vertical axis is in decibels and the horizontal axis is in megahertz. In the simulation shown in FIGS. 2A and 2B, the input signal represents a signal that is transmitted in a bandwidth of approximately 20 MHz that is centered in the figure. The wings of the signal represent noise outside of the transmission bandwidth. The noise level of the simulated input signal is approximately 80 dB below the signal in the transmission bandwidth, as shown in FIG. 2A. The simulation assumes a gain imbalance of 0.1 dB between the two paths of the LINC amplifier. In that case, the reconstructed signal shown in FIG. 2B has a noise floor that is only approximately 10 to 30 dB below the signal in the transmission bandwidth. Thus, the relatively small gain imbalance of 0.1 dB significantly degrades the quality of the reconstructed amplified signal.